This invention relates to a screening device and to apparatus employing such a screening device. The invention also relates to a method of recovering material from an underground location.
Screens are used for separating aggregate material according to size. One common form of screen is a grizzly which comprises a set of parallel bars in spaced apart relationship to provide elongate screen openings. In some situations, the screen openings in grizzlys can become clogged by material obstructing the openings. The obstructing material may be trapped in the screen openings or may simply accumulate on the intake side of the screen openings.
The present invention seeks to provide a screening device which has a screen and provision to remove obstructing material from screen openings within the screen.
The screening device according to the invention has particular application in the recovery of material from an underground location. In such an application, the screening device may be incorporated in a recovery head operable underground. Indeed, the recovery head incorporating the screening device may form part of an underground mining apparatus which has been devised particularly, although not necessarily exclusively, for mining in normally inaccessible locations including underground land-based formations such as deep leads, and deep marine alluvial deposits.
A deep lead typically comprises an ancient river valley through which an underground stream flows through an aquifer which contains metallic particles or heavy minerals below an overburden containing mud, sand and in some cases basalt. The metallic particles and/or heavy minerals in the stream bed are concentrated in the lower regions of sand and that part of the bedrock which has corroded to form a deposit known as xe2x80x9cwashxe2x80x9d. The wash may be of irregular composition, comprising a combination of sand, mud, and rocks and boulders of various sizes. To efficiently recover the metallic particles and/or heavy minerals, only wash material should be removed. Removal of wash material does, however, present considerable difficulties owing to its irregular composition and the uneven nature of the bedrock beneath it. It is the presence of the rocks and boulders in the wash material which presents a particular difficulty.
The inaccessible nature of the formation and in particular the depth underground as well as the irregular composition of the wash material precludes mining by way of a conventional dredging process.
Similarly, marine deposits may be too deep for conventional mining operations.
The present invention also seeks to provide a novel and useful system for recovering materials from underground locations.
According to one aspect of the invention there is provided a screening device comprising a screen having a first side for presentation to a body of material to be screened and a second side, a plurality of elongate screen openings in the screen extending between the first and second sides thereof, a plurality of tines mounted on support means disposed on the second side of the screen, each tine being movable along a path at least part of which includes one of the elongate screen openings whereby the tine is receivable in and movable along the elongate screen opening, the tine extending beyond the first side of the screen for at least part of the movement thereof along the screen opening.
The tines may serve one or more of various functions. A common function of the tines may be to clear obstructions from the screen openings to prevent clogging thereof of the screen openings. The obstructions may include obstructing material temporarily trapped in the screen openings and obstructing material accumulating on the first side of the screen. The tines serve to dislodge the trapped obstructing material or to move the obstructing material across the first face and out of the way of the screen openings.
Another function of the tines may be to fracture solid matter in the vicinity of the screening device. The solid matter may comprise rocks or other lumps of matter including a mass of consolidated material such as a deposit of clay. The action of the tines fractures the solid matter so that fragments thereof can either pass through the screen openings or be moved clear of the screen openings by the tines.
Still another function of the tines may be to provide traction to the apparatus in which the screening device according to the invention is fitted. Where such apparatus is required to move through a body of material such as an underground formation, the tines may engage the formation to provide traction to the apparatus.
Still another function of the tines may be to assist in creation of a slurry on the first side of the screen.
Still another function of the tines may be to agitate slurry material that has passed through the screen to maintain solids in suspension in the slurry.
Preferably, the support means is adapted to move each tine along a cyclical path. As each tine moves along the cyclical path, it may enter the respective screen opening from the second side of the screen, move along the screen opening, and then withdraw from the screen opening on the second side of the screen.
The support means for the tines may comprise a closed loop structure adapted for cyclical movement. The closed loop structure may include an endless chain or belt passing around end rollers, or a rotating drum. In this way, the tines are moved through the cyclical path, a part of which takes each tine along a respective one of the screen openings. Alternative arrangements for moving each tine through the cyclical path may comprise a fluid ram system, a walking beam mechanism or a lag and cam mechanism.
In one arrangement, a drive means may be provided for driving the closed loop structure for cyclical movement thereof. The drive means may comprise a hydraulic motor.
In another arrangement, the closed loop structure may be caused to move cyclically under the influence of the body of material acting on the tines.
The tines may be movably supported on the support means so as to be capable of deflection upon encountering an unmanagable obstruction such as a large boulder which can neither be moved nor fragmented. This allows the tines to pass the unmanagable obstruction without damage.
The tines may be resiliently movable on the support means so as to be yieldingly resist such deflection and to return to the undeflected condition after moving clear of the unmanagable obstruction.
In one arrangement, the screen may comprise a grizzly.
In another arrangement, the screen may comprise a wedge-wire screen. Such a screen includes supporting cross-members extending transversely of the wires. The cross-members span the screen openings and so provide obstructions to movement of the tines therealong. The tines must therefore be movably supported on the support means so as to be capable of deflection to pass the cross-members. For this purpose, each tine may comprise one of a set of tines mounted in a radial arrangement about an axis of rotation transverse to the cyclical path of movement of the set of tines. Each set of tines is arranged for step-by-step rotation so that upon one tine in the set encountering one of the cross-members, or indeed some other immovable obstruction, the set of tines is caused to rotate thereby deflecting the tine rearwardly to pass the obstructing cross-member. As the tine deflects, the set of tines indexes to position another one of the tines in the screen opening.
Sets of tines may be positioned in banks which extend across the screen. The various sets of tines in each bank may be mounted on a common hub which indexes the sets of tines in unison.
The screening device according to the first aspect of the invention has particular application in the recovery of material from an underground location. In such an application, the screening device may be incorporated in a recovery head according to a second aspect of the invention. The recovery head may be used for recovery of material from underground for various purposes such as mining operations, and tunnelling or other excavation operations. The recovery head comprises a body adapted for movement through an underground formation, a chamber provided within the body into which material to be recovered can enter after passing through the screen device and from where such material can be conveyed to a remote location.
Material entering the chamber of the recovery head is conveniently in the form of a slurry, the screen regulating the maximum size of solids within the slurry. The slurry is particularly convenient as it can be conveyed to a remote location, such as a ground surface, by a suction or pumping operation.
A conveying means may be provided for conveying the recovered material from the recovery head to the remote location.
The conveying means may comprise a pipe system extending between the recovery head and the remote location for conveyance of the recovered material. This is particularly suitable in circumstances where the recovered material is conveyed in a slurry form. The pipe system may be in the form of a pipe string.
In circumstances where the recovered material is not in slurry form, the conveying means may comprise a belt conveyor.
The pipe string may also be employed to deliver replacement material from the remote location to the underground zone. Alternatively, a separate pipe string or the like may be provided for such purpose. The replacement material is provided for replacing at least a portion of the material recovered from the underground zone.
The body of the recovery head may have a frontal surface which is tapered upwardly and rearwardly with respect to the normal direction of forward travel of the recovery head and which is presented to oncoming material as the recovery head advances through the underground zone. The screening device is preferably located in or otherwise associated with the frontal surface.
The slurry is preferably formed by agitating the mixture of solid materials and water present in the underground zone at which the recovery operation is taking place. It may be necessary to fragment consolidated materials, and in particular clay, in the underground zone in order to form the slurry and also allow the recovery head to move through the zone.
The introduction of water, preferably at high pressure, into the zone may also assist in formation of the slurry or indeed to form the slurry in circumstances where the underground stream is inactive.
The recovery head may also have means for disturbing the slurry in order to maintain heavy particles in suspension in the slurry. Such means may comprise means for vibrating the recovery head or at least a part thereof to agitate the slurry.
The frontal surface of the recovery head may define a leading edge at the lower end thereof which provides a blade to facilitate passage of the recovery head through the underground zone.
The recovery head may also have a re arward surface which tapers downwardly and rearwardly to a trailing edge. The trailing edge may provide a blade which facilitates passage of the recovery head through the underground zone on movement of the recovery head in the reverse direction. The rearward surface may have a second screening device according to the invention located in or otherwise associated with it.
The recovery head may have an upper surface extending between the frontal and rearward surfaces. The upper surface may also have a third screening device according to the invention located in or otherwise associated with it.
The tines of one or more of the various screening devices incorporated in the recovery head may provide traction for propelling the recovery head through the underground zone.
Means may be provided for discharging water under pressure into the region below the recovery head to establish a layer of water which provides a fluid cushion for providing support to the recovery head and thereby assisting its movement through the underground zone. Alternatively, an endless track may be provided on the underside of the recovery head to assist movement thereof through the underground zone. Similar endless tracks may be provided on side walls of the recovery head.
The recovery head may be provided with a control system incorporating sensors and controls for continuously observing and actively controlling all aspects of the underground mining operation. Such controls may include navigational controls, radiation sensors, and detection devices for detecting the structure of the deposit in which the recovery head is operating and in particular identifying and locating precious materials.
According to a fourth aspect of the invention there is provided apparatus for recovering material from an underground location, comprising a recovery head for receiving the material to be recovered and a conveying means for conveying the material from the recovery head to the remote location, the recovery head comprising a chamber for receiving material to be recovered, a screen associated with the chamber for screening material entering the chamber, the second having a first side from material being screened passes therethrough and a second side, a plurality of elongate screen openings in the screen extending between the first and second sides thereof, a plurality of tines mounted on support means disposed on the second side of the screen each tine being movable along a path at least part of which includes one of the elongate screen openings whereby the tine is receivable in and movable along the elongate screen opening, the tine extending beyond the first side of the screen for at least part of the movement thereof along the screen opening.
According to a fifth aspect of the invention there is provided a method of recovering material from an underground location, including the steps of: forming a slurry in a zone of an underground formation containing material to be recovered, screening the slurry to exclude constituents thereof above a predetermined size, conveying the screened material to a remote location, and delivering replacement material to the zone to replace at least a portion of the material conveyed therefrom.
The slurry may be formed by fragmenting consolidated material in the zone.